The present invention relates to emitter followers. In particular, the present invention relates to an apparatus and method that has a voltage gain and current gain that are comparable to a PNP emitter follower circuit that employs a PNP transistor with a large die area.
An emitter follower is a single transistor circuit that is arranged in a common-collector configuration. The transistor in the emitter follower can be an NPN transistor or a PNP transistor. An emitter follower has a voltage gain from the base of the transistor to the emitter that is close to one. The emitter follower is often used to provide a high current output since the transistor has a high current gain. Additionally, the emitter follower has a high input impedance and a low output impedance. An emitter follower is useful for isolating or buffering a high-resistance source such that a low-resistance load does not excessively load down the source.
A method and apparatus are directed to emulating an emitter follower with a small PNP transistor that is arranged in a PNP multiplier configuration. The PNP multiplier includes a PNP emitter follower and a current mirror. The PNP follower is coupled between the input and the output. A current mirror is coupled to the collector of the PNP follower such that mirror produces a current that is a scaled version of the collector current from the PNP follower. The current mirror is arranged to scale the PNP collector current by a factor of N. The effective output current from the PNP multiplier circuit corresponds to xcex2xc2x7IINxc2x7(N+1), where xcex2 corresponds to the large signal forward gain of the PNP follower. By multiplying the output current by a scaling factor, the effective forward gain of the PNP transistor is increased while utilizing a small geometry PNP device.